1. Field of the Invention
The present invention relates to a method for increasing the oxidation resistance of a Fe-Cr-Al alloy by forming a protective film of excellent oxidation resistance on the surface of said alloy.
2. Description of the Prior Art
In order to increase the oxidation resistance of a metal, it is generally known to form an oxidation-resistant protective film on the surface of the metal.
For example, an article "Oxide Structure of Stainless Steels under Controlled Oxygen Atmospheres" (Toshiyuki Yashiro, "Heat Treatment", Vol. 31, No. 4, pp. 205-211, 1991) and an article "Surface Modification of Stainless Steels Using Thermal Passivation" (Toshiyuki Yashiro, Keiichi Terashima, Taketomo Yamazaki, "Surface Technology", Vol. 41, No. 3, pp. 41-48, 1990) report a method for increasing the corrosion resistance of stainless steel or the like, which comprises heat-treating stainless steel or the like in a low-pressure oxygen or a controlled atmosphere to form a passive oxide film on the surface.
Also, Japanese Patent Publication No. 63148/1992 discloses a method for forming an alumina film on the surface of a TiAl intermetallic compound, which comprises placing said compound in an atmosphere having an oxygen partial pressure of 1.times.10.sup.-2 to 1.times.10.sup.-5 Pa at 900.degree.-1,050.degree. C. for 30 minutes to 100 hours to oxidize only Al selectively.
The above prior art is used for stainless steel or the like, or a TiAl intermetallic compound. When used for a Fe-Cr-Al alloy, however, the prior art has been unable to form a homogeneous protective film (an alumina film) because the pressure employed during film formation is too low (-10.sup.-2 Pa or lower).